Fuel-cell systems continue to offer much theoretical promise for the production of electricity. Proton exchange membrane (PEM) fuel cells have drawn much attention in the last decade, due to the potential for a high-efficiency and low-emission source of energy.
For many of the fuel-cell systems known in the art, the performance can be dramatically influenced by the PEM and its interaction with the fuel. Fuel permeability results in undesirable diffusion of the fuel from the anode to the cathode. For example, direct methanol fuel cells (DMFCs) currently suffer from fuel crossover, which limits the concentration of methanol that can be fed to the fuel cell. This limitation prevents the fuel cell from achieving higher current densities and power outputs due to poor utilization.
A DMFC feeds a methanol/water solution directly to the fuel cell (such as described in U.S. Pat. No. 5,599,638, for example). Nafion®, a perfluorinated sulphonic acid polymer, is often used as a PEM membrane material for fuel cells which operate at temperatures close to ambient. Nafion is typically employed as the PEM membrane in DMFCs. Other membranes of modified perfluorinated sulfonic acid polymers, polyhydrocarbon sulfonic acid polymers, and composites thereof, in addition to Nafion, are known.
The method of proton conduction in these systems requires that the membrane be hydrated, because protons are transported by the movement of hydronium ions (H3O+) through the membrane. The need for hydration often exacerbates the problem of fuel crossover, as PEM membranes are usually permeable to many fuels including methanol. When electric current flows through the fuel cell, the migrating hydronium will carry both water and the fuel (especially when it is highly miscible with water) across the membrane. The problem of fuel crossover limits the permissible fuel concentration.
Modification of the PEM membrane has been attempted to reduce the effect of crossover. One such system is described in U.S. Pat. No. 7,022,810 to Cornelius. In this patent, Cornelius describes a “hybrid inorganic-organic” membrane. Synthesis of the membrane is achieved by mixing an organic precursor of the proton-conducting polymer with an inorganic salt. The resulting membrane composition can be varied by altering the starting materials. These types of membranes reduce, but do not eliminate, fuel crossover while at the expense of proton conductivity which ultimately negatively impacts fuel cell performance.
There is a commercial need to reduce fuel crossover in fuel cells, thereby increasing fuel cell efficiency and power output. What is desired is an improved fuel cell, and methods to fabricate such a fuel cell, without chemical or physical modification to a desired PEM material itself. It would be further desirable for improved fuel cells to allow the use of fuels that would otherwise be incompatible with the desired PEM.